Power conversion device

ABSTRACT

A power conversion device includes: a power converter connected to an AC grid to which a load is connected; and a control circuit. The control circuit includes a harmonic compensation unit that includes a current command generation unit and a limit coefficient calculation unit and compensates for harmonic current contained in load current. The current command generation unit generates compensation current desired values for respective frequency components, and corrects the compensation current desired values using corresponding limit coefficients, to generate compensation current commands for respective frequency components. The limit coefficient calculation unit calculates each limit coefficient, on the basis of the compensation current desired value for each frequency component, and maximum voltage and maximum current that the power converter can output.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on PCT filing PCT/JP2020/011610, filedMar. 17, 2020, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to a power conversion device.

BACKGROUND ART

In an AC grid to which a load is connected, a power conversion device isconnected for compensating for harmonic currents. In a conventionalpower conversion device, harmonic detection means provided for eachharmonic order detects a harmonic component in load current separatelyon a harmonic-order basis, and a compensation current command value foreach harmonic is generated to perform output control of the powerconversion device. Thus, the compensation ratio can be easily set, and acompensation gain and a compensation capacitance limiter can be setindividually for each order, whereby it is possible to compensate foronly a harmonic for a specific order (see, for example, Patent Document1).

CITATION LIST Patent Document

-   Patent Document 1: Japanese Laid-Open Patent Publication No.    5-049172

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the conventional power conversion device, harmonic compensation canbe performed by setting a limit individually for each harmonic order,but in some cases, voltage and current that the power conversion devicecan output cannot be sufficiently used.

The present disclosure has been made to solve the above problem, and anobject of the present disclosure is to provide a power conversion devicethat effectively performs harmonic compensation for each harmonic orderwhile promoting usage of voltage and current that the power conversiondevice can output.

Solution to the Problems

A power conversion device according to the present disclosure includes:a power converter connected to an AC grid to which a load is connected;and a control circuit for performing output control of the powerconverter. The control circuit includes a harmonic compensation unit forcompensating for harmonic current contained in load current bycompensation current, and controls the compensation current in outputcurrent of the power converter. The harmonic compensation unit includesa current command generation unit and a limit coefficient calculationunit. The current command generation unit generates a compensationcurrent desired value for each of a plurality of frequency components,and corrects each compensation current desired value using acorresponding limit coefficient, to generate a compensation currentcommand for each frequency component. The limit coefficient calculationunit calculates each limit coefficient, on the basis of the compensationcurrent desired value for each of the plurality of frequency components,and maximum voltage and maximum current that the power converter is ableto output.

Effect of the Invention

The power conversion device according to the present disclosure makes itpossible to effectively perform harmonic compensation for each harmonicorder while promoting usage of voltage and current that the powerconversion device can output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a schematic configuration of a powerconversion device according to embodiment 1.

FIG. 2 shows an example of a circuit configuration of a power converteraccording to embodiment 1.

FIG. 3 shows another example of a circuit configuration of a powerconverter according to embodiment 1.

FIG. 4 shows another example of a circuit configuration of a powerconverter according to embodiment 1.

FIG. 5 shows an example of a circuit configuration of a converter cellin the power converter according to embodiment 1.

FIG. 6 shows another example of a circuit configuration of a convertercell in the power converter according to embodiment 1.

FIG. 7 shows another example of a circuit configuration of a powerconverter according to embodiment 1.

FIG. 8 shows a configuration of a current command generation unit of thepower conversion device according to embodiment 1.

FIG. 9 shows a configuration of a limit coefficient calculation unit ofthe power conversion device according to embodiment 1.

FIG. 10 shows a configuration of a voltage limitation value generationunit in the limit coefficient calculation unit according to embodiment1.

FIG. 11 shows a configuration of a current limitation value generationunit in the limit coefficient calculation unit according to embodiment1.

FIG. 12 shows a configuration of a coefficient calculating unit in thelimit coefficient calculation unit according to embodiment 1.

FIG. 13 shows a configuration of a current command generation unit of apower conversion device according to embodiment 2.

FIG. 14 shows a schematic configuration of a limit coefficientcalculation unit of the power conversion device according to embodiment2.

FIG. 15 shows a detailed configuration of the limit coefficientcalculation unit of the power conversion device according to embodiment2.

FIG. 16 is a block diagram showing a schematic configuration of a powerconversion device according to embodiment 3.

FIG. 17 shows a configuration of a current command generation unit ofthe power conversion device according to embodiment 3.

FIG. 18 shows a configuration of a limit coefficient calculation unit ofthe power conversion device according to embodiment 3.

FIG. 19 shows a configuration of a voltage limitation value generationunit in the limit coefficient calculation unit according to embodiment3.

FIG. 20 shows a configuration of a current limitation value generationunit in the limit coefficient calculation unit according to embodiment3.

FIG. 21 shows a configuration of a coefficient calculating unit in thelimit coefficient calculation unit according to embodiment 3.

FIG. 22 shows a configuration of a current command generation unit of apower conversion device according to embodiment 4.

FIG. 23 shows a schematic configuration of a limit coefficientcalculation unit of a power conversion device according to embodiment 4.

FIG. 24 shows a detailed configuration of the limit coefficientcalculation unit of the power conversion device according to embodiment4.

FIG. 25 is a block diagram showing a schematic configuration of a powerconversion device according to embodiment 5.

FIG. 26 shows a configuration of a voltage limitation value generationunit in a limit coefficient calculation unit according to embodiment 6.

FIG. 27 shows a configuration of a current limitation value generationunit in a limit coefficient calculation unit according to anotherexample of embodiment 6.

FIG. 28 shows a configuration of a circuit for calculating a voltagemaximum value of an AC grid according to embodiment 7.

DESCRIPTION OF EMBODIMENTS Embodiment 1

FIG. 1 is a block diagram showing a schematic configuration of a powerconversion device according to embodiment 1.

As shown in FIG. 1 , in a power system in which a load 2 is connected toa three-phase (U phase, V phase, W phase) AC grid 1, a power conversiondevice 100 is connected to the AC grid 1, in parallel to the load 2. Thepower conversion device 100 includes a power converter 10 and a controlcircuit 20 for performing output control of the power converter 10, andthe power converter 10 is connected to the AC grid 1 via a transformer11.

A voltage detector 3 and current detectors 4, 5 are provided fordetecting voltage and currents at respective parts. The voltage detector3 detects interconnection point voltage Vs (Vsu, Vsv, Vsw) which isvoltage of the AC grid 1 interconnected with the power converter 10. Thecurrent detector 4 detects load current iload (iloadu, iloadv, iloadw)flowing from the AC grid 1 to the load 2, and the current detector 5detects output current i (iu, iv, iw) outputted from the power converter10 to the AC grid 1.

The control circuit 20 performs switching control of switching elementsin the power converter 10 on the basis of input information(interconnection point voltage Vs, load current iload, output current i)from the voltage detector 3 and the current detectors 4, 5, therebyperforming output control of the power converter 10.

The control circuit 20 includes an active power control unit 21, areactive power control unit 22, a harmonic compensation unit 23, acurrent control unit 24, and a pulse width modulation (PWM) control unit25.

In the present embodiment, a case of outputting a fundamentalpositive-phase-sequence component (first-order positive-phase-sequencecomponent) and also compensating for harmonics will be described. Here,frequency components other than a fundamental positive-phase-sequencecomponent, i.e., including a fundamental negative-phase-sequencecomponent, are treated as harmonics.

The active power control unit 21 and the reactive power control unit 22generate an active current command id1 p and a reactive current commandiq1 p as a first current command for the fundamentalpositive-phase-sequence component. The active power control unit 21 andthe reactive power control unit 22 each receive values ofinterconnection point voltage Vs and output current i from the voltagedetector 3 and the current detector 5. Then, the active power controlunit 21 generates a d-axis current command as the active current commandid1 p for the fundamental positive-phase-sequence component on the basisof active power of the AC grid 1. The reactive power control unit 22generates a q-axis current command as the reactive current command iq1 pfor the fundamental positive-phase-sequence component on the basis ofreactive power of the AC grid 1.

The harmonic compensation unit 23 includes a current command generationunit 30 and a limit coefficient calculation unit 40, and generatescompensation current commands 31 for compensating for a plurality offrequency components that are harmonic currents by compensationcurrents.

The current command generation unit 30 receives a value of the loadcurrent iload from the current detector 4 and a limit coefficient Kgenerated for each frequency component by the limit coefficientcalculation unit 40. Then, the current command generation unit 30generates a compensation current desired value 32 for each of theplurality of frequency components on the basis of the load currentiload, and corrects the compensation current desired value 32 using thelimit coefficient K, to generate the compensation current command 31.

The limit coefficient calculation unit 40 receives the active currentcommand id1 p and the reactive current command iq1 p from the activepower control unit 21 and the reactive power control unit 22, and thecompensation current desired values 32 from the current commandgeneration unit 30, and generates the limit coefficient K for eachfrequency component on the basis of the received values.

The current control unit 24 receives the active current command id1 pand the reactive current command iq1 p from the active power controlunit 21 and the reactive power control unit 22, and the compensationcurrent commands 31 from the harmonic compensation unit 23, andgenerates a voltage command 24 a on the basis of an output currentcommand obtained by adding the received values. On the basis of thevoltage command 24 a, the PWM control unit 25 performs comparison usinga carrier wave, to generate a gate signal 25 a for switching elements inthe power converter 10.

FIG. 2 shows an example of a circuit configuration of the powerconverter 10.

As shown in FIG. 2 , the power converter 10 is configured as a modularmultilevel converter (MMC) of a double-star-connection type. In thiscase, the power converter 10 includes a plurality of converter cells 12connected in series in each of upper and lower arms for each of threephases connected by double star connection which is an example of starconnection, and includes arm reactors Lp, Ln on the positive side andthe negative side.

The power converter may have a circuit configuration of a powerconverter 10A shown in FIG. 3 . As shown in FIG. 3 , the power converter10A is configured as a MMC of a delta-connection type. The powerconverter 10A includes a plurality of converter cells 12 connected inseries in each of arms for each of three phases connected by deltaconnection, and includes arm reactors L.

Further, the power converter may have a circuit configuration of a powerconverter 10B shown in FIG. 4 . As shown in FIG. 4 , the power converter10B is configured as a MMC of a single-star-connection type which isanother example of star connection. The power converter 10B includes aplurality of converter cells 12 connected in series in each of arms foreach of three phases connected by single star connection, and includesarm reactors L.

FIG. 5 shows an example of a circuit configuration of the converter cell12 in the power converter 10, 10A.

As shown in FIG. 5 , the converter cell 12 is configured as ahalf-bridge circuit having two switching elements 13 to which diodes areconnected in antiparallel, and a DC capacitor 14. The switching elements13 are self-turn-off switching elements such as insulated-gate bipolartransistors (IGBT), for example. In the converter cell 12, a connectionpoint between the two switching elements 13 and a DC terminal on thenegative side are used as output ends, and two kinds of voltages, i.e.,zero voltage and voltage across the DC capacitor 14 are outputtedthrough switching of the switching elements 13.

The converter cell may have a circuit configuration of a converter cell12A shown in FIG. 6 . As shown in FIG. 6 , the converter cell 12A isconfigured as a full-bridge circuit in which two legs each formed byseries-connection of two switching elements 13 to which diodes areconnected in antiparallel are connected in parallel with a DC capacitor14. In the converter cell 12A, a connection point between the twoswitching elements 13 in each leg is used as an output end, and threekinds of voltages, i.e., zero voltage and positive and negative voltagesacross the DC capacitor 14 are outputted through switching of theswitching elements 13.

The power converter of the power conversion device 100 may have acircuit configuration of a power converter 10C shown in FIG. 7 . Asshown in FIG. 7 , the power converter 10C is configured as a three-phase2-level inverter in which three legs each formed by series connection oftwo switching elements 13 to which diodes are connected in antiparallelare connected in parallel with the DC capacitor 14. A connection pointbetween the two switching elements 13 in each leg is used as an outputend for each phase, and positive or negative voltage of the DC capacitor14 is outputted for each phase through switching of the switchingelements 13.

The power conversion device 100 is used for a static synchronouscompensator (STATCOM), an active filter, a high voltage direct current(HVDC) system for performing high-voltage DC power transmission, or thelike.

The power converter 10 configured as the MMC of a double-star-connectiontype and the power converter 10C configured as a three-phase 2-levelinverter are used for an active filter, a STATCOM, a HVDC system, or thelike. The active filter is a device for compensating for a harmoniccomponent in the AC grid 1, and the STATCOM is a device for compensatingfor reactive power in the AC grid 1. The HVDC system is used for a gridof which the power transmission distance is long, and is a system inwhich power of an AC grid is forward converted to high-voltage DC powerto be transmitted and then the DC power is inversely converted to ACpower to be transmitted to another AC grid.

The power converter 10A configured as the MMC of a delta-connection typeis used for a distribution-STATCOM (D-STATCOM) for power distribution.The D-STATCOM performs reactive power compensation in a low-order gridclose to a load.

The power converter 10B configured as the MMC of asingle-star-connection type is used for an active filter, a STATCOM, orthe like.

In a case where the power conversion device 100 is used for a devicesuch as the STATCOM or the HVDC system that outputs a fundamentalpositive-phase-sequence component with highest priority given thereto,it is necessary to perform harmonic compensation while considering thefundamental positive-phase-sequence component outputted from the powerconverter 10. In addition, since voltage that the power converter 10 canoutput is limited, it is necessary to appropriately set a limiter forcompensation current included in the output current i of the powerconverter 10, also for preventing overmodulation.

Hereinafter, the configuration and operation of the harmoniccompensation unit 23 will be described in detail. The power conversiondevice 100 according to the present embodiment outputs a fundamentalpositive-phase-sequence component and also compensates for harmonicswith a priority set for each frequency component.

FIG. 8 shows a configuration of the current command generation unit 30of the harmonic compensation unit 23.

The current command generation unit 30 includes a plurality ofcoordinate conversion units 33 that perform 3-phase/2-phase rotationalcoordinate conversion to extract a plurality of frequency componentsthat are harmonic compensation targets, on the basis of the load currentiload, and further includes a plurality of filters 34 and a plurality ofmultipliers 35.

The three-phase load currents iload are subjected to rotationalcoordinate conversion by each of the plurality of coordinate conversionunits 33, to be converted to DC currents for the plurality of frequencycomponents. The plurality of coordinate conversion units 33 are set withdifferent phases for coordinate conversion. For example, in a case ofperforming conversion to dq-axis currents id1 n, iq1 n of a fundamentalnegative-phase-sequence component, coordinate conversion is performedwith a negative fundamental component phase, and in a case of performingconversion to dq-axis currents id5 p, iq5 p of a fifth-orderpositive-phase-sequence component, coordinate conversion is performedwith a positive fifth-order component phase. In this case, thefundamental negative-phase-sequence component, the fifth-orderpositive-phase-sequence component, a fifth-order negative-phase-sequencecomponent, . . . , a kth-order positive-phase-sequence component, and akth-order negative-phase-sequence component are compensation targets,and dq-axis currents (id1 n, iq1 n), (id5 p, iq5 p), (id5 n, iq5 n), . .. , (idkp, iqkp), and (idkn, iqkn) are generated.

The dq-axis currents generated for each frequency component pass thefilter 34 by which harmonic components are removed from DC current sothat only the frequency component that is the compensation target isextracted. Thus, for the respective frequency components, thecompensation current desired values 32, i.e., (id1 nf, iq1 nf), (id5 pf,iq5 pf), (id5 nf, iq5 nf), . . . , (idkpf, iqkpf), and (idknf, iqknf)are generated.

Then, at the multiplier 35, the compensation current desired value 32for each frequency component is multiplied by the limit coefficient K(K1 n, K5 p, K5 n, . . . , Kkp, and Kkn) generated for each frequencycomponent by the limit coefficient calculation unit 40. Thus, for therespective frequency components, the compensation current commands 31,i.e., (id1 n*, iq1 n*), (id5 p*, iq5 p*), (id5 n*, iq5 n*), . . . ,(idkp*, iqkp*), and (idkn*, iqkn*) are generated.

FIG. 9 shows a configuration of the limit coefficient calculation unit40 of the harmonic compensation unit 23.

The limit coefficient calculation unit 40 includes a voltage limitationvalue generation unit 50, a current limitation value generation unit 60,and a coefficient calculating unit 70. The limit coefficient calculationunit 40 receives the active current command id1 p and the reactivecurrent command iq1 p from the active power control unit 21 and thereactive power control unit 22, and the compensation current desiredvalues 32 from the current command generation unit 30, and generates thelimit coefficients K (K1 n, K5 p, K5 n, . . . , Kkp, and Kkn) for therespective frequency components on the basis of the received values.

FIG. 10 shows a configuration of the voltage limitation value generationunit 50 in the limit coefficient calculation unit 40, and FIG. 11 showsa configuration of the current limitation value generation unit 60 inthe limit coefficient calculation unit 40. FIG. 12 shows a configurationof the coefficient calculating unit 70 in the limit coefficientcalculation unit 40.

Regarding harmonics to be compensated for by the harmonic compensationunit 23, a priority of harmonic compensation is set for each of theplurality of frequency components, and in the limit coefficientcalculation unit 40, the voltage limitation value generation unit 50 andthe current limitation value generation unit 60 generate voltage upperlimit values Vh1 to Vhm and current upper limit values Ih1 to Ihm forthe respective frequency components, in the order of the priorities.

As shown in FIG. 9 and FIG. 10 , in the voltage limitation valuegeneration unit 50, a grid voltage maximum value Vsmax is subtractedfrom maximum voltage Vmax that the power converter 10 can output,thereby calculating voltage Vhmax that can be used for control of thepower converter 10.

The voltage limitation value generation unit 50 receives the activecurrent command id1 p and the reactive current command iq1 p, andcompensation current desired values 32 a ((id1 nf, iq1 nf), (id5 pf, iq5pf), (id5 nf, iq5 nf), . . . , (idkpf, iqkpf)) excluding the kth-ordernegative-phase-sequence component having the lowest priority among thecompensation current desired values 32 from the current commandgeneration unit 30.

In the voltage limitation value generation unit 50, the received d-axisand q-axis currents of each frequency component are squared bymultipliers 51, and a square root of a sum obtained by adding thesquared values is calculated by a square root calculator 52, therebygenerating current amplitude values |I1 p|, |I1 nf|, |I5 pf|, |I5 nf|, .. . , |Ikpf|. Then, at constant multipliers 53, 54, each currentamplitude value is multiplied by a constant corresponding to the ordernumber of the current and a fundamental impedance XL of a reactor in thetransformer 11, thereby calculating voltage needed for outputting eachcurrent amplitude value, i.e., voltage to be allocated for eachfrequency component.

Further, the voltage limitation value generation unit 50 subtracts thevoltage to be allocated for the fundamental positive-phase-sequencecomponent from the voltage Vhmax, and then performs limiter processingfor making a negative value be zero by a limiter 55, thereby calculatingthe voltage upper limit value Vh1 for the frequency component for whichharmonic compensation is performed with the priority set at the firstrank, in this case, the fundamental negative-phase-sequence component.Then, from the voltage upper limit value Vh1 before the limiterprocessing, the voltage to be allocated for the fundamentalnegative-phase-sequence component is subtracted, and the resultant valueis similarly subjected to limiter processing, thus calculating thevoltage upper limit value Vh2 for the frequency component of which thepriority is at the second rank, in this case, the fifth-orderpositive-phase-sequence component.

Similarly, for each frequency component that is the compensation target,the voltage to be allocated for the frequency component is calculated,and this voltage is subtracted from the voltage upper limit value beforelimiter processing, to calculate the voltage upper limit value for thefrequency component of which the priority is at the next rank.

Thus, the voltage upper limit values Vh1, Vh2, Vh3, Vh4, . . . , Vhm aregenerated in the order of priorities from the first rank. The generatedvoltage upper limit values are inputted to the coefficient calculatingunit 70.

In a case where the voltage value inputted to the limiter 55 is anegative value, the output voltage of the power converter 10 has alreadyexceeded the usable voltage, and therefore the voltage value is made tobe zero by limiter processing and calculation of voltage upper limitvalues is not performed for the subsequent frequency components.

As shown in FIG. 9 and FIG. 11 , the current limitation value generationunit 60 receives the active current command id1 p and the reactivecurrent command iq1 p, and the compensation current desired values 32((id1 nf, iq1 nf), (id5 pf, iq5 pf), (id5 nf, iq5 nf), . . . , (idkpf,iqkpf), and (idknf, iqknf)) from the current command generation unit 30.

Regarding the received dq-axis currents of each frequency component, thed-axis and q-axis currents are squared by multipliers 61, and a squareroot of a sum obtained by adding the squared values is calculated by asquare root calculator 62, thereby generating current amplitude values|I1 p|, |I1 nf|, |I5 pf|, |I5 nf|, . . . , |Ikpf|, and |Iknf|. Thecurrent amplitude values are currents to be allocated for the respectivefrequency components.

Further, the current limitation value generation unit 60 subtracts thecurrent amplitude value |I1 p| which is the current to be allocated forthe fundamental positive-phase-sequence component from maximum currentImax that the power converter 10 can output, and then performs limiterprocessing for making a negative value be zero by a limiter 63, thuscalculating the current upper limit value Ih1 for the frequencycomponent for which harmonic compensation is performed with the priorityset at the first rank, in this case, the fundamentalnegative-phase-sequence component. Then, from the current upper limitvalue Ih1 before the limiter processing, the current amplitude value |I1nf| which is the current to be allocated for the fundamentalnegative-phase-sequence component is subtracted, and the resultant valueis similarly subjected to limiter processing, thus calculating thecurrent upper limit value Ih2 for the frequency component of which thepriority is at the second rank, in this case, the fifth-orderpositive-phase-sequence component.

Similarly, for each frequency component that is the compensation target,the current (current amplitude value) to be allocated for the frequencycomponent is calculated, and this current is subtracted from the currentupper limit value before limiter processing, to calculate the currentupper limit value for the frequency component of which the priority isat the next rank.

Thus, the current limitation value generation unit 60 generates thecurrent upper limit values Ih1, Ih2, Ih3, Ih4, . . . , Ihm for therespective frequency components that are the compensation targets, inthe order of priorities from the first rank. Then, the generated currentupper limit values Ih1, Ih2, Ih3, Ih4, . . . , Ihm and the generatedcurrent amplitude values |I1 nf|, |I5 pf|, |I5 nf|, . . . , |Ikpf|, and|Iknf| as second current values are outputted from the currentlimitation value generation unit 60 and inputted to the coefficientcalculating unit 70.

In a case where the current value inputted to the limiter 63 is anegative value, the output current of the power converter 10 has alreadyexceeded the usable current, and therefore the current value is made tobe zero by limiter processing and calculation of current upper limitvalues is not performed for the subsequent frequency components. For thekth-order negative-phase-sequence component having the lowest priority,limiter processing is not performed after the current amplitude value|Iknf| is calculated, and only the current amplitude value |Iknf| isoutputted.

As shown in FIG. 9 and FIG. 12 , the coefficient calculating unit 70receives the voltage upper limit values Vh1, Vh2, Vh3, Vh4, . . . , Vhm,the current upper limit values Ih1, Ih2, Ih3, Ih4, . . . , Ihm, and thecurrent amplitude values |I1 nf|, |I5 pf|, |I5 nf|, . . . , |Ikpf|, and|Iknf| generated for the respective frequency components that are thecompensation targets.

Then, for each frequency component, the limit coefficient is calculatedusing the voltage upper limit value, the current upper limit value, andthe current amplitude value. For example, for the fundamentalnegative-phase-sequence component of which the priority is at the firstrank, the fundamental impedance XL of the reactor in the transformer 11is multiplied by a constant corresponding to the order number (in thiscase, 1) at a constant multiplier 71, thereby calculating an impedancecorresponding to the order number. Then, the voltage upper limit valueVh1 is divided by the calculated impedance at a divider 72, therebycalculating a first current value Ih1 a which is a current limitationvalue based on the voltage upper limit value Vh1.

The calculated first current value Ih1 a, the calculated current upperlimit value Ih1, and the calculated current amplitude value |I1 nf| areinputted to a minimum value extractor 73, and the minimum valueextractor 73 extracts the minimum value among the three current values.This minimum value is divided by the current amplitude value |I1 nf| ata divider 74, thereby calculating the limit coefficient K1 n which isthe ratio of the minimum value to the current amplitude value |I1 nf|.

Similarly, also regarding the other frequency components, the minimumvalues are extracted among the first current values Ih2 a, . . . , Ihmacalculated from the voltage upper limit values Vh2, . . . , Vhm and theimpedances XL based on the frequencies, the current upper limit valuesIh2, . . . , Ihm, and the current amplitude values |I5 pf|, . . . ,|Iknf| for the respective frequency components, and the minimum valuesare divided by the current amplitude values |I5 pf|, . . . , |Iknf|,thereby calculating the limit coefficients K5 p, . . . , Kkn.

The compensation current desired values 32 for respective frequencycomponents are multiplied by the limit coefficients K (K1 n, K5 p, . . ., Kkn) generated as described above, that is, the compensation currentdesired values 32 are corrected using the limit coefficients K, wherebythe compensation current commands 31 are generated.

As described above, in the present embodiment, the harmonic compensationunit 23 in the control circuit 20 generates the compensation currentdesired value 32 for each of the plurality of frequency components onthe basis of the load current iload, and calculates each limitcoefficient K on the basis of the compensation current desired value 32,and the maximum voltage Vmax and the maximum current Imax that the powerconverter 10 can output. Then, each compensation current desired value32 is corrected using the corresponding limit coefficient K, therebygenerating the compensation current command 31 for each frequencycomponent.

Thus, while sufficient usage of the voltage and the current that thepower converter 10 can output is promoted, the limit coefficient can becalculated individually in accordance with the order number of thecompensation current, and thus it is possible to effectively performharmonic compensation for each harmonic order. In addition, the limitcoefficients can be calculated in real time in accordance with theoperation condition of the power conversion device 100 and thus it ispossible to perform control with high reliability and high accuracy.

In the present embodiment, in the harmonic compensation unit 23, for thefrequency component that is the compensation target (e.g., first-ordernegative-phase-sequence component), the voltage upper limit value Vh1and the current upper limit value Ih1 are calculated, the minimum valueis extracted among the first current value Ih1 a based on the voltageupper limit value Vh1, the current upper limit value Ih1, and thecurrent amplitude value |I1 nf| of the compensation current desiredvalue 32, and the minimum value is divided by the current amplitudevalue |I1 nf|, thereby calculating the limit coefficient K1 n. Thus, itis possible to generate the compensation current command 31 such thatthe usable voltage and current are not exceeded and can be sufficientlyused, for each frequency component that is the compensation target.

In the present embodiment, in the harmonic compensation unit 23, apriority is set for each frequency component that is the compensationtarget, and the voltage upper limit value (e.g., Vh1) and the currentupper limit value (e.g., Ih1) are calculated in a descending order ofthe priorities. Thus, the voltage and the current that can be used forthe power converter 10 can be allocated from a frequency componenthaving a higher priority, whereby harmonic compensation can beeffectively performed.

In the present embodiment, the harmonic compensation unit 23 calculatesthe voltage upper limit value (Vh1) and the current upper limit value(Ih1) for the frequency component of which the priority is at the firstrank, on the basis of the maximum voltage Vmax and the maximum currentImax that the power converter 10 can output and the first currentcommand (active current command id1 p and reactive current command iq1p) for the fundamental positive-phase-sequence component. Then, for eachfrequency component that is the compensation target, the harmoniccompensation unit 23 calculates voltage and current to be allocated forthe frequency component, and subtracts the calculated voltage andcurrent from the voltage upper limit value (e.g., Vh1) and the currentupper limit value (e.g., Ih1), respectively, thereby calculating thevoltage upper limit value (Vh2) and the current upper limit value (Ih2)for the frequency component of which the priority is at the next rank(e.g., second rank).

Thus, while the voltage and current that can be used for the powerconverter 10 are assuredly and accurately allocated from a frequencycomponent having a higher priority, the voltage upper limit values(e.g., Vh1) and the current upper limit values (e.g., Ih1) can becalculated.

In the above embodiment, for the fundamental negative-phase-sequencecomponent, the priority of harmonic compensation is set at the firstrank. However, even in a case of another priority rank, the compensationcurrent command 31 can be generated in the same manner and the sameeffects are obtained.

In the above description, the fundamental impedance XL is the impedancein the transformer 11, but impedances of the reactors L, Lp, Ln in thepower converter 10 may be added thereto.

In the above embodiment, the harmonic compensation unit 23 generates thecompensation current desired value 32 for each of the plurality offrequency components on the basis of the load current iload andgenerates the compensation current command 31. However, the compensationcurrent desired value 32 may be generated from another control system.For example, on the basis of the interconnection point voltage Vs, thecompensation current desired value 32 may be generated so as to reduce aharmonic component of the interconnection point voltage Vs.

Embodiment 2

In the above embodiment 1, the case where priorities of harmoniccompensation are different among a plurality of frequency components,has been shown. In embodiment 2, a case where priorities are set at thesame rank among a plurality of frequency components, will be describedbelow. Also in embodiment 2, a schematic configuration of the powerconversion device 100 is the same as that in the above embodiment 1shown in FIG. 1 , and while a fundamental positive-phase-sequencecomponent is outputted, harmonic compensation is performed for eachfrequency component.

Hereinafter, the configuration and operation of the harmoniccompensation unit 23 according to embodiment 2 will be described indetail.

As in the above embodiment 1, the harmonic compensation unit 23 includesa current command generation unit 30A and a limit coefficientcalculation unit 40A, and generates the compensation current commands 31for compensating for harmonic currents by compensation currents. In thepresent embodiment, a priority of harmonic compensation for thefundamental negative-phase-sequence component is set at the first rank,and priorities for the other components are all set at the same rank.

The current command generation unit 30A receives the value of the loadcurrent iload from the current detector 4 and the limit coefficient Kgenerated for each frequency component by the limit coefficientcalculation unit 40A. Then, the current command generation unit 30Agenerates the compensation current desired value 32 for each of theplurality of frequency components on the basis of the load currentiload, and corrects the compensation current desired value 32 using thelimit coefficient K, to generate the compensation current command 31.

FIG. 13 shows a configuration of the current command generation unit 30Aaccording to embodiment 2, and FIG. 14 shows a schematic configurationof the limit coefficient calculation unit 40A.

The current command generation unit 30A receives the value of the loadcurrent iload and the limit coefficient K (K1 n, Kh) generated for eachfrequency component by the limit coefficient calculation unit 40A. Inthis case, since a priority of harmonic compensation for the fundamentalnegative-phase-sequence component is set at the first rank, andpriorities for the other components are all set at the same rank, thelimit coefficients Kh for the frequency components other than thefundamental negative-phase-sequence component are all equal to eachother.

As in the above embodiment 1, the current command generation unit 30Aincludes the plurality of coordinate conversion units 33 that perform3-phase/2-phase rotational coordinate conversion to extract a pluralityof frequency components that are harmonic compensation targets, on thebasis of the load current iload, and further includes the plurality offilters 34 and the plurality of multipliers 35.

The three-phase load currents iload are subjected to rotationalcoordinate conversion by each of the plurality of coordinate conversionunits 33, to be converted to DC currents for the plurality of frequencycomponents. The plurality of coordinate conversion units 33 are set withdifferent phases for coordinate conversion. For example, in a case ofperforming conversion to the dq-axis currents id1 n, iq1 n of thefundamental negative-phase-sequence component, coordinate conversion isperformed with a negative fundamental component phase, and in a case ofperforming conversion to the dq-axis currents id5 p, iq5 p of thefifth-order positive-phase-sequence component, coordinate conversion isperformed with a positive fifth-order component phase. In this case, thefundamental negative-phase-sequence component, the fifth-orderpositive-phase-sequence component, the fifth-ordernegative-phase-sequence component, . . . , the kth-orderpositive-phase-sequence component, and the kth-ordernegative-phase-sequence component are compensation targets, and dq-axiscurrents (id1 n, iq1 n), (id5 p, iq5 p), (id5 n, iq5 n), . . . , (idkp,iqkp), (idkn, iqkn) are generated. The dq-axis currents generated foreach frequency component pass the filter 34 by which harmonic componentsare removed from DC current so that only the frequency component that isthe compensation target is extracted. Thus, for the respective frequencycomponents, the compensation current desired values 32, i.e., (id1 nf,iq1 nf), (id5 pf, iq5 pf), (id5 nf, iq5 nf), . . . , (idkpf, iqkpf),(idknf, iqknf) are generated.

Then, at the multiplier 35, the compensation current desired value 32for each frequency component is multiplied by the limit coefficient K(K1 n, Kh) generated for each frequency component by the limitcoefficient calculation unit 40A. Thus, for the respective frequencycomponents, the compensation current commands 31, i.e., (id1 n*, iq1n*), (id5 p*, iq5 p*), (id5 n*, iq5 n*), . . . , (idkp*, iqkp*), and(idkn*, iqkn*) are generated. In this case, the limit coefficients K (K1n, Kh) generated for the respective frequency components are the sameexcept for the fundamental negative-phase-sequence component, andtherefore two kinds of limit coefficients are used.

FIG. 15 shows a detailed configuration of the limit coefficientcalculation unit 40A.

As shown in FIG. 15 , the limit coefficient calculation unit 40Areceives the active current command id1 p and the reactive currentcommand iq1 p, and the compensation current desired values 32 ((id1 nf,iq1 nf), (id5 pf, iq5 pf), (id5 nf, iq5 nf), . . . , (idkpf, iqkpf), and(idknf, iqknf)) from the current command generation unit 30A.

In the limit coefficient calculation unit 40A, the received d-axis andq-axis currents of each frequency component are squared by multipliers81, and a square root of a sum obtained by adding the squared values iscalculated by a square root calculator 82, thereby generating currentamplitude values |I1 p|, |I1 nf|, |I5 pf|, |I5 nf|, . . . , |Ikpf|, and|Iknf|.

In addition, the limit coefficient calculation unit 40A subtracts thegrid voltage maximum value Vsmax from the maximum voltage Vmax that thepower converter 10 can output, thereby calculating the voltage Vhmaxthat can be used for control of the power converter 10. In addition, ata constant multiplier 83, the current amplitude value |I1 p| of thefundamental positive-phase-sequence component is multiplied by thefundamental impedance XL of the reactor in the transformer 11, therebycalculating voltage needed for outputting the current amplitude value|I1 p|, i.e., voltage to be allocated for the fundamentalpositive-phase-sequence component.

Then, the limit coefficient calculation unit 40A subtracts the voltageto be allocated for the fundamental positive-phase-sequence componentfrom the voltage Vhmax, thereby calculating the voltage upper limitvalue Vh1 for the frequency component for which harmonic compensation isperformed with the priority set at the first rank, in this case, thefundamental negative-phase-sequence component.

Then, the voltage upper limit value Vh1 is divided by the fundamentalimpedance XL at a divider 84, thereby calculating a first current valueIh1 b which is a current limitation value based on the voltage upperlimit value Vh1.

In addition, the limit coefficient calculation unit 40A subtracts thecurrent amplitude value |I1 p| which is current to be allocated for thefundamental positive-phase-sequence component from the maximum currentImax that the power converter 10 can output, thereby calculating acurrent upper limit value Ih1 c for the fundamentalnegative-phase-sequence component.

The calculated first current value Ih1 b, the calculated current upperlimit value Ih1 c, and the calculated current amplitude value |I1 nf| ofthe fundamental negative-phase-sequence component as the second currentvalue are inputted to a minimum value extractor 85, and the minimumvalue extractor 85 extracts the minimum value among the three currentvalues. This minimum value is divided by the current amplitude value |I1nf| at a divider 86, thereby calculating the limit coefficient K1 nwhich is the ratio of the minimum value to the current amplitude value|I1 nf|.

The limit coefficient Kh for the frequency component group of which thepriorities are at the second rank or below is calculated as follows.

In the limit coefficient calculation unit 40A, the sum of the currentamplitude values |I1 p| and |I1 nf| of the fundamentalpositive-phase-sequence component and the fundamentalnegative-phase-sequence component is multiplied by the fundamentalimpedance XL of the reactor in the transformer 11, at a constantmultiplier 91, thereby calculating voltage needed for outputting thecurrent amplitude value |I1 p| and the current amplitude value |I1 nf|,i.e., voltage to be allocated for the fundamentalpositive-phase-sequence component and the fundamentalnegative-phase-sequence component.

Then, the limit coefficient calculation unit 40A subtracts the voltageto be allocated for the fundamental positive-phase-sequence componentand the fundamental negative-phase-sequence component from the voltageVhmax, thereby calculating a voltage upper limit value Vhh for thefrequency component group of which the priorities are at the second rankor below. Then, the voltage upper limit value Vhh is divided by thefundamental impedance XL at a divider 92, thereby calculating a currentlimitation value 92 a based on the voltage upper limit value Vhh.

In addition, regarding the current amplitude values |I5 pf|, |I5 nf|, .. . , |Ikpf|, and |Iknf| other than the fundamentalpositive-phase-sequence component and the fundamentalnegative-phase-sequence component, an amplitude sum Ih which is thetotal sum of these amplitude values is calculated as the second currentvalue, by a total sum calculator 87. Further, the current amplitudevalues |I5 pf|, |I5 nf|, . . . , |Ikpf|, and |Iknf| are multiplied byconstants corresponding to the respective order numbers at constantmultipliers 88, and the total sum thereof is calculated as a thirdcurrent value Iha by a total sum calculator 89. The third current valueIha is the total sum of the current amplitude values imparted withweights based on the respective frequencies.

Then, a value 90 a obtained by dividing the amplitude sum Ih (secondcurrent value) by the third current value Iha at a divider 90 ismultiplied by the current limitation value 92 a based on the voltageupper limit value Vhh at a multiplier 93, thereby calculating a firstcurrent value Ihha which is a current limitation value based on thevoltage upper limit value Vhh for the frequency component group of whichthe priorities are at the second rank or below.

In addition, in the limit coefficient calculation unit 40A, the currentamplitude values |I1 p| and |I1 nf| which are currents to berespectively allocated for the fundamental positive-phase-sequencecomponent and the fundamental negative-phase-sequence component aresequentially subtracted from the maximum current Imax that the powerconverter 10 can output, thereby calculating a current upper limit valueIhh for the frequency component group of which the priorities are at thesecond rank or below.

The calculated first current value Ihha, the calculated current upperlimit value Ihh, and the calculated amplitude sum Ih (second currentvalue) are inputted to a minimum value extractor 94, and the minimumvalue extractor 94 extracts the minimum value among the three currentvalues. This minimum value is divided by the amplitude sum Ih at adivider 95, thereby calculating the limit coefficient Kh which is theratio of the minimum value to the amplitude sum Ih.

The compensation current desired values 32 for the respective frequencycomponents are multiplied by the limit coefficients K (K1 n, Kh)generated as described above, that is, the compensation current desiredvalues 32 are corrected using the limit coefficients K, whereby thecompensation current commands 31 are generated.

Also in the present embodiment, as in the above embodiment 1, whilesufficient usage of the voltage and the current that the power converter10 can output is promoted, the limit coefficient can be calculated inaccordance with each order number of the compensation current, and thusit is possible to effectively perform harmonic compensation for eachharmonic order. In addition, the limit coefficients K can be calculatedin real time in accordance with the operation condition of the powerconversion device 100 and thus it is possible to perform control withhigh reliability and high accuracy.

In addition, in the present embodiment, for the plurality of frequencycomponents of which the priorities are at the second rank or below, thecompensation current commands 31 can be generated using the same limitcoefficient K without particularly setting different priorities.

Further, for example, in a case of desiring to preferentially compensatefor a specific frequency component because of resonance of atransmission line or the like, the priority for that frequency componentmay be set at the first rank, and priorities for the other frequencycomponents may be set at the same rank, whereby it is possible toeffectively perform harmonic compensation so as to specialize in thespecific frequency component.

In the above embodiment, priorities of harmonic compensation for thefrequency components at the second rank or below are all set at the samerank. However, by combining the method of the above embodiment 1, thesame ranks of priorities may be set for a plurality of frequencycomponent groups and thus a total of three or more different limitcoefficients K may be generated to generate the compensation currentcommands 31, whereby control with a high degree of freedom can berealized.

Embodiment 3

In the above embodiments 1 and 2, also for the frequency component ofwhich the priority of harmonic compensation is at the first rank, thevoltage upper limit value Vh1 and the current upper limit values Ih1,Ih1 c are calculated and the limit coefficient K1 n is calculated.

However, the present disclosure is not limited thereto. FIG. 16 is ablock diagram showing a schematic configuration of a power conversiondevice according to embodiment 3.

As in the above embodiment 1, the power conversion device 100 includesthe power converter 10 and the control circuit 20, and the controlcircuit 20 performs switching control of switching elements in the powerconverter 10 on the basis of input information (interconnection pointvoltage Vs, load current iload, output current i) from the voltagedetector 3 and the current detectors 4, 5, thereby performing outputcontrol of the power converter 10.

The control circuit 20 includes the active power control unit 21, thereactive power control unit 22, a harmonic compensation unit 23A, thecurrent control unit 24, and the PWM control unit 25. The harmoniccompensation unit 23A includes the current command generation unit 30and a limit coefficient calculation unit 40B, and generates thecompensation current commands 31 for compensating for harmonic currentsby compensation currents.

In embodiment 3, the active current command id1 p and the reactivecurrent command iq1 p from the active power control unit 21 and thereactive power control unit 22 are inputted only to the current controlunit 24. The limit coefficient calculation unit 40B receives thecompensation current desired values 32 from the current commandgeneration unit 30, and generates the limit coefficient K for eachfrequency component on the basis of the compensation current desiredvalues 32. The other configurations are the same as those in the aboveembodiment 1 shown in FIG. 1 .

FIG. 17 shows a configuration of the current command generation unit 30of the harmonic compensation unit 23A. The current command generationunit 30 is the same as that in the above embodiment 1 shown in FIG. 8 ,and operates in the same manner to generate the compensation currentdesired value 32 and generate the compensation current command 31, foreach frequency component.

FIG. 18 shows a configuration of the limit coefficient calculation unit40B of the harmonic compensation unit 23A.

The limit coefficient calculation unit 40B includes a voltage limitationvalue generation unit 50B, a current limitation value generation unit60B, and a coefficient calculating unit 70B. The limit coefficientcalculation unit 40B receives the compensation current desired values 32from the current command generation unit 30, and generates the limitcoefficient K (K1 n, K5 p, K5 n, . . . , Kkp, and Kkn) for eachfrequency component.

FIG. 19 shows a configuration of the voltage limitation value generationunit 50B in the limit coefficient calculation unit 40B, and FIG. 20shows a configuration of the current limitation value generation unit60B in the limit coefficient calculation unit 40B. FIG. 21 shows aconfiguration of the coefficient calculating unit 70B in the limitcoefficient calculation unit 40.

Regarding harmonics to be compensated for by the harmonic compensationunit 23A, a priority of harmonic compensation is set for each of theplurality of frequency components, and in this case, the priority forthe fundamental negative-phase-sequence component is set at the firstrank. In the limit coefficient calculation unit 40B, the voltagelimitation value generation unit 50B and the current limitation valuegeneration unit 60B generate the voltage upper limit values Vh2 to Vhmand the current upper limit values Ih2 to Ihm for the respectivefrequency components, in a descending order of priorities from thesecond rank.

As shown in FIG. 18 and FIG. 19 , the voltage limitation valuegeneration unit 50B receives compensation current desired values 32 b((id5 pf, iq5 pf), (id5 nf, iq5 nf), . . . , (idkpf, iqkpf)) excludingthe fundamental negative-phase-sequence component of which the priorityis at the first rank and the kth-order negative-phase-sequence componenthaving the lowest priority among the compensation current desired values32 from the current command generation unit 30.

In the voltage limitation value generation unit 50B, the received d-axisand q-axis currents of each frequency component are squared by themultipliers 51, and a square root of a sum obtained by adding thesquared values is calculated by the square root calculator 52, therebygenerating the current amplitude values |I5 pf|, |I5 nf|, . . . ,|Ikpf|. Then, at the constant multipliers 53, 54, each current amplitudevalue is multiplied by a constant corresponding to the order number ofthe current and the fundamental impedance XL of the reactor in thetransformer 11, thereby calculating voltage needed for outputting eachcurrent amplitude value, i.e., voltage to be allocated for eachfrequency component.

In addition, in the voltage limitation value generation unit 50B, thegrid voltage maximum value Vsmax is subtracted from the maximum voltageVmax that the power converter 10 can output, thereby calculating thevoltage Vhmax that can be used for control of the power converter 10.Then, the sum of a current maximum value I1 pmax of the fundamentalpositive-phase-sequence component and a current maximum value I1 nmax ofthe fundamental negative-phase-sequence component is multiplied by thefundamental impedance XL of the reactor in the transformer 11, at theconstant multiplier 54, thereby calculating voltage for the fundamentalpositive-phase-sequence component and the fundamentalnegative-phase-sequence component. The calculated voltage is subtractedfrom the voltage Vhmax, thereby calculating the voltage upper limitvalue Vh2 for the frequency component for which harmonic compensation isperformed with the priority set at the second rank, in this case, thefifth-order positive-phase-sequence component.

Further, the voltage limitation value generation unit 50B subtracts thevoltage to be allocated for the fifth-order positive-phase-sequencecomponent from the voltage upper limit value Vh2 for the fifth-orderpositive-phase-sequence component, and then performs limiter processingfor making a negative value be zero by the limiter 55, therebycalculating the voltage upper limit value Vh3 for the frequencycomponent for which harmonic compensation is performed with the priorityset at the third rank, in this case, the fifth-ordernegative-phase-sequence component. Similarly, for each frequencycomponent that is the compensation target, voltage to be allocated forthe frequency component is calculated, and this voltage is subtractedfrom the voltage upper limit value before limiter processing, tocalculate the voltage upper limit value for the frequency component ofwhich the priority is at the next rank.

Thus, the voltage upper limit values Vh2, Vh3, Vh4, . . . , Vhm aregenerated in the order of priorities from the second rank. The generatedvoltage upper limit values are inputted to the coefficient calculatingunit 70B.

In a case where the voltage value inputted to the limiter 55 is anegative value, the output voltage of the power converter 10 has alreadyexceeded the usable voltage, and therefore the voltage value is made tobe zero by limiter processing and calculation of voltage upper limitvalues is not performed for the subsequent frequency components.

As shown in FIG. 18 and FIG. 20 , the current limitation valuegeneration unit 60B receives compensation current desired values 32 c((id5 pf, iq5 pf), (id5 nf, iq5 nf), . . . , (idkpf, iqkpf), and (idknf,iqknf)) excluding the fundamental negative-phase-sequence component ofwhich the priority is at the first rank among the compensation currentdesired values 32 from the current command generation unit 30.

In the current limitation value generation unit 60B, the received d-axisand q-axis currents of each frequency component are squared by themultipliers 61, and a square root of a sum obtained by adding thesquared values is calculated by the square root calculator 62, therebygenerating the current amplitude values |I5 pf|, |I5 nf|, . . . ,|Ikpf|, and |Iknf|. The current amplitude values are currents to beallocated for the respective frequency components, in this case, thefrequency components of which the priorities are at the second rank orbelow.

Further, the current limitation value generation unit 60B subtracts thecurrent maximum value I1 pmax of the fundamental positive-phase-sequencecomponent and the current maximum value I1 nmax of the fundamentalnegative-phase-sequence component from the maximum current Imax that thepower converter 10 can output, thereby calculating the current upperlimit value Ih2 for the frequency component for which harmoniccompensation is performed with the priority set at the second rank, inthis case, the fifth-order positive-phase-sequence component.

Then, from the current upper limit value Ih2, the current amplitudevalue |I5 pf| which is the current to be allocated for the fifth-orderpositive-phase-sequence component is subtracted, and the resultant valueis subjected to limiter processing for making a negative value be zeroby the limiter 55, thereby calculating the current upper limit value Ih3for the frequency component of which the priority is at the third rank,in this case, the fifth-order negative-phase-sequence component.

Similarly, for each frequency component that is the compensation target,the current (current amplitude value) to be allocated for the frequencycomponent is calculated, and this current is subtracted from the currentupper limit value before limiter processing, to calculate the currentupper limit value for the frequency component of which the priority isat the next rank.

Thus, for the respective frequency components that are the compensationtargets, the current limitation value generation unit 60B generates thecurrent upper limit values Ih2, Ih3, Ih4, . . . , Ihm in the order ofpriorities from the second rank. Then, the generated current upper limitvalues Ih2, Ih3, Ih4, . . . , Ihm and the current amplitude values |I5pf|, I5 nf|, . . . , |Ikpf|, |Iknf| as second current values areoutputted from the current limitation value generation unit 60B andinputted to the coefficient calculating unit 70B.

In a case where the current value inputted to the limiter 63 is anegative value, the output current of the power converter 10 has alreadyexceeded the usable current, and therefore the current value is made tobe zero by limiter processing and calculation of current upper limitvalues is not performed for the subsequent frequency components. For thekth-order negative-phase-sequence component having the lowest priority,limiter processing is not performed after the current amplitude value|Iknf| is calculated, and only the current amplitude value |Iknf| isoutputted.

As shown in FIG. 18 and FIG. 21 , the coefficient calculating unit 70Breceives the fundamental negative-phase-sequence component (id1 nf, iq1nf) of which the priority is at the first rank among the compensationcurrent desired values 32 from the current command generation unit 30,and further receives the voltage upper limit values Vh2, Vh3, Vh4, . . ., Vhm, the current upper limit values Ih2, Ih3, Ih4, . . . , Ihm, andthe current amplitude values |I5 pf|, |I5 nf|, . . . , |Ikpf|, and|Iknf| generated for the respective frequency components of which thepriorities are at the second rank or below.

In the coefficient calculating unit 70B, the d-axis and q-axis currentsfor the fundamental negative-phase-sequence component (id1 nf, iq1 nf)of which the priority is at the first rank are squared by multipliers75, and a square root of a sum obtained by adding the squared values iscalculated by a square root calculator 76, thereby calculating thecurrent amplitude value |I1 nf|. Then, the current amplitude value |I1nf| is subjected to limiter processing with a predetermined limitationvalue, e.g., I1 nmax, by a limiter 77 a, and also subjected to limiterprocessing for avoiding zero by a limiter 77 b, and the output of thelimiter 77 a is divided by the output of the limiter 77 b, at a divider78. Thus, the limit coefficient K1 n for the fundamentalnegative-phase-sequence component is generated.

In addition, for each frequency component of which the priorities are atthe second rank or below, the coefficient calculating unit 70Bcalculates the limit coefficient using the voltage upper limit value,the current upper limit value, and the current amplitude value, as inthe above embodiment 1. For example, for the fifth-orderpositive-phase-sequence component, the fundamental impedance XL of thereactor in the transformer 11 is multiplied by a constant correspondingto the order number (in this case, 5) at the constant multiplier 71,thereby calculating an impedance corresponding to the order number.Then, the voltage upper limit value Vh2 is divided by the calculatedimpedance at the divider 72, thereby calculating the first current valueIh2 a which is the current limitation value based on the voltage upperlimit value Vh2.

The calculated first current value Ih2 a, the calculated current upperlimit value Ih2, and the calculated current amplitude value |I5 pf| areinputted to the minimum value extractor 73, and the minimum valueextractor 73 extracts the minimum value among the three current values.This minimum value is divided by the current amplitude value |I5 pf| atthe divider 74, thereby calculating the limit coefficient K5 p which isthe ratio of the minimum value to the current amplitude value |I5 pf|.

Similarly, also regarding the other frequency components of which thepriorities are at the second rank or below, the minimum values areextracted among the first current values Ih3 a, . . . , Ihma calculatedfrom the voltage upper limit values Vh3, . . . , Vhm and the impedancesXL based on the frequencies, the current upper limit values Ih3, . . . ,Ihm, and the current amplitude values |I5 nf|, . . . , |Iknf| for therespective frequency components, and the minimum values are divided bythe current amplitude values |I5 nf|, . . . , |Iknf|, therebycalculating the limit coefficients K5 n, . . . , Kkn.

The compensation current desired values 32 for the respective frequencycomponents are multiplied by the limit coefficients K (K1 n, K5 p, . . ., Kkn) generated as described above, that is, the compensation currentdesired values 32 are corrected using the limit coefficients K, wherebythe compensation current commands 31 are generated.

Also in the present embodiment, as in the above embodiment 1, whilesufficient usage of the voltage and the current that the power converter10 can output is promoted, the limit coefficient can be calculated inaccordance with each order number of the compensation current, and thusit is possible to effectively perform harmonic compensation for eachharmonic order. In addition, the limit coefficient K can be calculatedin real time in accordance with the operation condition of the powerconversion device 100 and thus it is possible to perform control withhigh reliability and high accuracy.

In a case of desiring to preferentially compensate for a specificfrequency component, the limit coefficient (K1 n) for individuallycompensating for that frequency component may be generated, andpriorities may be set for the other frequency components, to generatethe limit coefficients K5 p, . . . , Kkn using the voltage upper limitvalues and the current upper limit values as in the above embodiment 1.Thus, it is possible to perform harmonic compensation so as tospecialize in the specific frequency component, and also for the otherfrequency components, it is possible to accurately perform harmoniccontrol in accordance with priorities.

Embodiment 4

In the above embodiment 3, different limit coefficients K are calculatedfor the frequency components of which the priorities are at the secondrank or below. However, as in the above embodiment 2, for the frequencycomponents of which the priorities are at the second rank or below, thepriorities may be set at the same rank and thus an equal limitcoefficient K may be calculated.

As in embodiment 3 shown in FIG. 16 , the power conversion device 100includes the power converter 10 and the control circuit 20, and thecontrol circuit 20 performs switching control of switching elements inthe power converter 10 on the basis of input information(interconnection point voltage Vs, load current iload, output current i)from the voltage detector 3 and the current detectors 4, 5, therebyperforming output control of the power converter 10.

The control circuit 20 includes the active power control unit 21, thereactive power control unit 22, the harmonic compensation unit 23A, thecurrent control unit 24, and the PWM control unit 25. The harmoniccompensation unit 23A includes the current command generation unit 30Aand a limit coefficient calculation unit 40C, and generates thecompensation current commands 31 for compensating for harmonic currentsby compensation currents.

In embodiment 4, as in the above embodiment 3, the active currentcommand id1 p and the reactive current command iq1 p from the activepower control unit 21 and the reactive power control unit 22 areinputted only to the current control unit 24. The limit coefficientcalculation unit 40C receives the compensation current desired values 32from the current command generation unit 30A, and generates the limitcoefficient K for each frequency component on the basis of thecompensation current desired values 32. The other configurations are thesame as those in the above embodiment 1.

As in the above embodiment 1, the harmonic compensation unit 23Aincludes the current command generation unit 30A and the limitcoefficient calculation unit 40C, and generates the compensation currentcommands 31 for compensating for harmonic currents by compensationcurrents. In embodiment 4, a priority of harmonic compensation for thefundamental negative-phase-sequence component is set at the first rank,and priorities for the other components are all set at the same rank.

The current command generation unit 30A receives the value of the loadcurrent iload from the current detector 4 and the limit coefficient Kgenerated for each frequency component by the limit coefficientcalculation unit 40C. Then, the current command generation unit 30Agenerates the compensation current desired value 32 for each of theplurality of frequency components on the basis of the load currentiload, and corrects the compensation current desired value 32 using thelimit coefficient K, to generate the compensation current command 31.

FIG. 22 shows a configuration of the current command generation unit 30Aof the harmonic compensation unit 23A, and FIG. 23 shows a configurationof the limit coefficient calculation unit 40C of the harmoniccompensation unit 23A.

The current command generation unit 30A is the same as that in the aboveembodiment 2 shown in FIG. 13 , and operates in the same manner togenerate the compensation current desired value 32 and generate thecompensation current command 31, for each frequency component. Then, atthe multiplier 35, the compensation current desired value 32 for eachfrequency component is multiplied by the limit coefficient K (K1 n, Kh)generated for each frequency component by the limit coefficientcalculation unit 40C. Thus, for the respective frequency components, thecompensation current commands 31, i.e., (id1 n*, iq1 n*), (id5 p*, iq5p*), (id5 n*, iq5 n*), . . . , (idkp*, iqkp*), and (idkn*, iqkn*) aregenerated. In this case, the limit coefficients K (K1 n, Kh) generatedfor the respective frequency components are the same except for thefundamental negative-phase-sequence component, and therefore two kindsof limit coefficients are used.

FIG. 24 shows a detailed configuration of the limit coefficientcalculation unit 40C.

As shown in FIG. 24 , the limit coefficient calculation unit 40Creceives the compensation current desired values 32 ((id1 nf, iq1 nf),(id5 pf, iq5 pf), (id5 nf, iq5 nf), . . . , (idkpf, iqkpf), and (idknf,iqknf)) from the current command generation unit 30A.

In the limit coefficient calculation unit 40C, the received d-axis andq-axis currents of each frequency component are squared by themultipliers 81, and a square root of a sum obtained by adding thesquared values is calculated by the square root calculator 82, therebygenerating the current amplitude values |I1 nf|, |I5 pf|, |I5 nf|, . . ., |Ikpf|, and |Iknf|.

In addition, the limit coefficient calculation unit 40C generates thelimit coefficient K1 n for the fundamental negative-phase-sequencecomponent, in the same manner as in the above embodiment 3. That is, thecurrent amplitude value |I1 nf| is subjected to limiter processing witha predetermined limitation value, e.g., I1 nmax, by the limiter 77 a,and also subjected to limiter processing for avoiding zero by thelimiter 77 b, and the output of the limiter 77 a is divided by theoutput of the limiter 77 b, at the divider 78. Thus, the limitcoefficient K1 n for the fundamental negative-phase-sequence componentis generated.

The limit coefficient Kh for the frequency component group of which thepriorities are at the second rank or below is calculated as follows.

The limit coefficient calculation unit 40C subtracts the grid voltagemaximum value Vsmax from the maximum voltage Vmax that the powerconverter 10 can output, thereby calculating voltage Vhmax that can beused for control of the power converter 10. Then, the sum of the currentmaximum value I1 pmax of the fundamental positive-phase-sequencecomponent and the current maximum value I1 nmax of the fundamentalnegative-phase-sequence component is multiplied by the fundamentalimpedance XL of the reactor in the transformer 11, at the constantmultiplier 91, thereby calculating voltage for the fundamentalpositive-phase-sequence component and the fundamentalnegative-phase-sequence component. The calculated voltage is subtractedfrom the voltage Vhmax, thereby calculating the voltage upper limitvalue Vhh for the frequency component group of which the priorities areat the second rank or below. Then, the voltage upper limit value Vhh isdivided by the fundamental impedance XL at the divider 92, therebycalculating the current limitation value 92 a based on the voltage upperlimit value Vhh.

In addition, regarding the current amplitude values |I5 pf|, |I5 nf|, .. . , |Ikpf|, and |Iknf| other than the fundamentalpositive-phase-sequence component and the fundamentalnegative-phase-sequence component, the amplitude sum Ih which is thetotal sum of these amplitude values is calculated as the second currentvalue at the total sum calculator 87. Further, the current amplitudevalues |I5 pf|, |I5 nf|, . . . , |Ikpf|, |Iknf| are multiplied byconstants corresponding to the respective order numbers at the constantmultipliers 88, and the total sum thereof is calculated as the thirdcurrent value Iha at the total sum calculator 89. The third currentvalue Iha is the total sum of the current amplitude values imparted withweights based on the respective frequencies.

It is noted that, regarding frequency components for each order, apositive-phase-sequence component and a negative-phase-sequencecomponent are present, and therefore the case where the sum of thepositive-phase-sequence component and the negative-phase-sequencecomponent for the same order is calculated and then the total sumthereof is calculated, has been shown.

Then, the value 90 a obtained by dividing the amplitude sum Ih (secondcurrent value) by the third current value Iha at the divider 90 ismultiplied by the current limitation value 92 a based on the voltageupper limit value Vhh at the multiplier 93, thereby calculating thefirst current value Ihha which is the current limitation value based onthe voltage upper limit value Vhh for the frequency component group ofwhich the priorities are at the second rank or below.

In addition, in the limit coefficient calculation unit 40C, the currentmaximum value I1 pmax of the fundamental positive-phase-sequencecomponent and the current maximum value I1 nmax of the fundamentalnegative-phase-sequence component are sequentially subtracted from themaximum current Imax that the power converter 10 can output, therebycalculating the current upper limit value Ihh for the frequencycomponent group of which the priorities are at the second rank or below.

The calculated first current value Ihha, the calculated current upperlimit value Ihh, and the calculated amplitude sum Ih (second currentvalue) are inputted to the minimum value extractor 94, and the minimumvalue extractor 94 extracts the minimum value among the three currentvalues. This minimum value is divided by the amplitude sum Ih at thedivider 95, thereby calculating the limit coefficient Kh which is theratio of the minimum value to the amplitude sum Ih.

The compensation current desired values 32 for the respective frequencycomponents are multiplied by the limit coefficients K (K1 n, Kh)generated as described above, that is, the compensation current desiredvalues 32 are corrected using the limit coefficients K, whereby thecompensation current commands 31 are generated.

Also in the present embodiment, as in the above embodiment 1, whilesufficient usage of the voltage and the current that the power converter10 can output is promoted, the limit coefficient can be calculated inaccordance with each order number of the compensation current, and thusit is possible to effectively perform harmonic compensation for eachharmonic order. In addition, the limit coefficients K can be calculatedin real time in accordance with the operation condition of the powerconversion device 100 and thus it is possible to perform control withhigh reliability and high accuracy.

In addition, in the present embodiment, for the plurality of frequencycomponents of which the priorities are at the second rank or below, thecompensation current commands 31 can be generated using the same limitcoefficient K without particularly setting different priorities.

In addition, since the limit coefficient (K1 n) for preferentiallycompensating for a specific frequency component individually isgenerated, it is possible to perform harmonic compensation so as tospecialize in the specific frequency component.

Embodiment 5

The above embodiments have shown the power conversion device in whichthe active current command id1 p and the reactive current command iq1 pas the first current command for the fundamental positive-phase-sequencecomponent are generated separately from the compensation currentcommands 31. However, the harmonic compensation unit may generate acurrent command including the first current command for the fundamentalpositive-phase-sequence component.

FIG. 25 is a block diagram showing a schematic configuration of a powerconversion device 100A according to embodiment 5. It is noted that theAC grid 1, the load 2, and the detectors 3, 4, 5 for voltage andcurrents are the same as those in the above embodiment 1.

As shown in FIG. 25 , a control circuit 20A includes a harmoniccompensation unit 230, the current control unit 24, and the PWM controlunit 25.

The harmonic compensation unit 230 includes a current command generationunit 300 and a limit coefficient calculation unit 40D. The currentcommand generation unit 300 receives the values of the interconnectionpoint voltage Vs from the voltage detector 3, the output current i fromthe current detector 5, and the load current iload from the currentdetector 4, and the limit coefficient K generated for each frequencycomponent by the limit coefficient calculation unit 40D. Then, thecurrent command generation unit 300 generates a compensation currentdesired value 320 for each of the plurality of frequency components onthe basis of the load current iload, and generates an output currentcommand 310 including a compensation current command obtained bycorrecting the compensation current desired value 320 using the limitcoefficient K and the first current command for the fundamentalpositive-phase-sequence component.

The limit coefficient calculation unit 40D receives the compensationcurrent desired values 320 from the current command generation unit 300,and generates the limit coefficient K for each frequency component onthe basis of the received values.

The configuration of the power conversion device 100A shown in FIG. 25is applicable also to a case of outputting only harmonic compensationcurrents to perform harmonic compensation, as in an active filter or thelike. In this case, the output current command 310 outputted from theharmonic compensation unit 230 coincides with the compensation currentcommand.

Embodiment 6

In the above embodiments, in the case of configuring the power converter10 as a MMC, the interphase balance in output voltages of the convertercells 12 is preferentially controlled. In the case of the MMC of thesingle-star-connection type as shown in FIG. 4 , zero-phase-sequencevoltage Vzero is outputted for control of the interphase balance in theoutput voltages of the converter cells 12.

In embodiment 6, a case of using the power converter 10B configured asthe MMC of the single-star-connection type and outputtingzero-phase-sequence voltage Vzero for interphase balance control in thepower conversion device 100 of the above embodiment 1, will bedescribed.

FIG. 26 shows a configuration of a voltage limitation value generationunit in a limit coefficient calculation unit according to embodiment 6.

As shown in FIG. 26 , in a voltage limitation value generation unit 50Ain the limit coefficient calculation unit 40, the grid voltage maximumvalue Vsmax is subtracted from the maximum voltage Vmax that the powerconverter 10B can output, to calculate the voltage Vhmax that can beused for control of the power converter 10B, and further, the value ofthe zero-phase-sequence voltage Vzero is subtracted therefrom. Then,voltage to be allocated for each of the frequency components includingthe fundamental positive-phase-sequence component is subtracted, and asin the above embodiment 1, the voltage upper limit values Vh1, Vh2, . .. , Vhm are generated in a descending order of priorities.

The other control configurations are the same as those in the aboveembodiment 1.

Thus, the generated voltage upper limit values Vh1, Vh2, . . . , Vhmbecome voltages corrected by the zero-phase-sequence voltage Vzero.Therefore, also in the power conversion device 100 that outputs thezero-phase-sequence voltage Vzero to perform interphase balance control,the voltage and the current that the power converter 10B can output canbe sufficiently used, the limit coefficient can be calculated inaccordance with each order number of the compensation current, andharmonic compensation can be effectively performed for each harmonicorder.

In the above description, the case of application to the aboveembodiment 1 has been shown. However, the above configuration isapplicable also to the other embodiments, whereby the same effects areobtained.

In the above embodiment, the case of the power converter 10B configuredas the MMC of the single-star-connection type has been described.Meanwhile, in a case of the power converter 10, 10A configured as theMMC of the double-star-connection type or the MMC of thedelta-connection type as shown in FIG. 2 or FIG. 3 , circulation currentIcir is outputted for interphase balance control.

FIG. 27 shows a configuration of a current limitation value generationunit in a limit coefficient calculation unit according to anotherexample of embodiment 6. In this case, in the power conversion device100 of the above embodiment 1, the power converter 10, 10A configured asthe MMC of the double-star-connection type or the delta-connection typeis used, and the circulation current Icir is outputted for interphasebalance control.

As shown in FIG. 27 , a current limitation value generation unit 60A inthe limit coefficient calculation unit 40 subtracts the value of thecirculation current Icir from the maximum current Imax that the powerconverter 10, 10A can output. Then, current to be allocated for each ofthe frequency components including the fundamentalpositive-phase-sequence component is subtracted, and as in the aboveembodiment 1, the current upper limit values Ih1, Ih2, . . . , Ihm aregenerated in a descending order of priorities.

The other control configurations are the same as those in the aboveembodiment 1.

Thus, the generated current upper limit values Ih1, Ih2, . . . , Ihmbecome currents corrected by the circulation current Icir. Therefore,also in the power conversion device 100 that outputs the circulationcurrent Icir to perform interphase balance control, the voltage and thecurrent that the power converter 10, 10A can output can be sufficientlyused, the limit coefficient can be calculated in accordance with eachorder number of the compensation current, and harmonic compensation canbe effectively performed for each harmonic order.

Also in this case, the above configuration is applicable to not only theabove embodiment 1 but also the other embodiments, whereby the sameeffects are obtained.

Embodiment 7

In the above embodiments, regarding the grid voltage maximum valueVsmax, if it is necessary to consider distortion of the grid voltage atthe time of grid voltage disturbance, the grid voltage maximum valueVsmax can be generated considering voltage distortion for a desiredfrequency component.

FIG. 28 shows a configuration of a circuit for calculating the gridvoltage maximum value Vsmax which is the voltage maximum value of the ACgrid 1, according to embodiment 7.

As shown in FIG. 28 , the three-phase interconnection point voltages Vsare subjected to rotational coordinate conversion by each of a pluralityof coordinate conversion units 101, to be converted to DC voltages forthe plurality of frequency components. The plurality of coordinateconversion units 101 are set with different phases for coordinateconversion. For example, in a case of performing conversion to thedq-axis voltages Vd1 p, Vq1 p for the fundamentalpositive-phase-sequence component, coordinate conversion is performedwith a positive fundamental component phase, and in a case of performingconversion to the dq-axis voltages Vd1 n, Vq1 n for the fundamentalnegative-phase-sequence component, coordinate conversion is performedwith a negative fundamental component phase.

In this case, as frequency components for which voltage distortion isconsidered, the fundamental positive-phase-sequence component, thefundamental negative-phase-sequence component, the fifth-orderpositive-phase-sequence component, the fifth-ordernegative-phase-sequence component, . . . , the kth-orderpositive-phase-sequence component, and the kth-ordernegative-phase-sequence component are targeted, and the respectivedq-axis voltages (Vd1 n, Vq1 n), (Vd5 p, Vq5 p), (Vd5 n, Vq5 n), . . . ,(Vdkp, Vqkp), and (Vdkn, Vqkn) are generated.

The dq-axis voltages generated for each frequency component pass afilter 102 by which harmonic components are removed from DC voltage.Then, regarding the values of the dq-axis voltages after the filterprocessing, the d-axis and q-axis voltages for each frequency componentare squared by multipliers 103, and a square root of a sum obtained byadding the squared values is calculated by the square root calculator104, thereby generating voltage amplitude values |V1 p|, |V1 n|, |V5 p|,|V5 n|, . . . , |Vkp|, and |Vkn| for the respective frequencycomponents. The total sum of the generated voltage amplitude values iscalculated by a total sum calculator 105, thereby generating the gridvoltage maximum value Vsmax.

Thus, also in the case where voltage distortion occurs in the AC grid 1at the time of grid voltage disturbance, the grid voltage maximum valueVsmax can be reliably calculated, the voltage and the current that thepower converter 10 can output can be sufficiently used, and harmoniccompensation can be effectively performed.

In calculation for the grid voltage maximum value Vsmax consideringvoltage distortion, as long as the fundamental positive-phase-sequencecomponent is included, the other frequency components may be anyfrequency components.

Although the disclosure is described above in terms of various exemplaryembodiments and implementations, it should be understood that thevarious features, aspects, and functionality described in one or more ofthe individual embodiments are not limited in their applicability to theparticular embodiment with which they are described, but instead can beapplied, alone or in various combinations to one or more of theembodiments of the disclosure.

It is therefore understood that numerous modifications which have notbeen exemplified can be devised without departing from the scope of thepresent disclosure. For example, at least one of the constituentcomponents may be modified, added, or eliminated. At least one of theconstituent components mentioned in at least one of the preferredembodiments may be selected and combined with the constituent componentsmentioned in another preferred embodiment.

DESCRIPTION OF THE REFERENCE CHARACTERS

-   -   1 AC grid    -   2 load    -   10, 10A, 10B, 10C power converter    -   12, 12A converter cell    -   20, 20A control circuit    -   23, 23A, 230 harmonic compensation unit    -   30, 30A, 300 current command generation unit    -   31 compensation current command    -   32, 32 a, 32 b, 32 c, 320 compensation current desired value    -   40, 40A, 40B, 40C, 40D limit coefficient calculation unit    -   100, 100A power conversion device    -   310 output current command    -   iload load current    -   Imax maximum current    -   Ih1 to Ihm, Ih1 c, Ihh current upper limit value    -   Ih1 a to Ihma, Ih1 b, Ihha first current value    -   |I1 nf| to |Iknf|, Ih second current value    -   Iha third current value    -   id1 p active current command    -   iq1 p reactive current command    -   Icir circulation current    -   K, Kh, K1 n to Kkn limit coefficient    -   Vmax maximum voltage    -   Vh1 to Vhm, Vhh voltage upper limit value    -   Vsmax grid voltage maximum value    -   Vzero zero-phase-sequence voltage    -   XL fundamental impedance

The invention claimed is:
 1. A power conversion device comprising: apower converter connected to an AC grid to which a load is connected;and a control circuitry to perform output control of the powerconverter, wherein the control circuitry includes a harmoniccompensation circuitry to compensate for harmonic current contained inload current by compensation current, and controls the compensationcurrent in output current of the power converter, and the harmoniccompensation circuitry includes a current command generation circuitrywhich generates a compensation current desired value for each of aplurality of frequency components, and corrects each compensationcurrent desired value using a corresponding limit coefficient, togenerate a compensation current command for each frequency component,and a limit coefficient calculation circuitry which calculates eachlimit coefficient, on the basis of the compensation current desiredvalue for each of the plurality of frequency components, and maximumvoltage and maximum current that the power converter is able to output.2. The power conversion device according to claim 1, wherein theharmonic compensation circuitry generates the compensation currentdesired value for each of the plurality of frequency components on thebasis of the load current.
 3. The power conversion device according toclaim 2, wherein the harmonic compensation circuitry multiplies eachcompensation current desired value by the corresponding limitcoefficient, to generate the compensation current command for eachfrequency component.
 4. The power conversion device according to claim2, wherein for a frequency component that is a compensation target, theharmonic compensation circuitry calculates a voltage upper limit valueand a current upper limit value, extracts a minimum value among a firstcurrent value calculated from the voltage upper limit value and animpedance based on the frequency, the current upper limit value, and asecond current value based on an amplitude of the compensation currentdesired value, and divides the minimum value by the second currentvalue, to calculate the limit coefficient.
 5. The power conversiondevice according to claim 4, wherein the harmonic compensationcircuitry, with priorities set for respective frequency components thatare compensation targets, calculates the voltage upper limit value andthe current upper limit value in a descending order of the priorities.6. The power conversion device according to claim 1, wherein theharmonic compensation circuitry multiplies each compensation currentdesired value by the corresponding limit coefficient, to generate thecompensation current command for each frequency component.
 7. The powerconversion device according to claim 6, wherein for a frequencycomponent that is a compensation target, the harmonic compensationcircuitry calculates a voltage upper limit value and a current upperlimit value, extracts a minimum value among a first current valuecalculated from the voltage upper limit value and an impedance based onthe frequency, the current upper limit value, and a second current valuebased on an amplitude of the compensation current desired value, anddivides the minimum value by the second current value, to calculate thelimit coefficient.
 8. The power conversion device according to claim 7,wherein the harmonic compensation circuitry, with priorities set forrespective frequency components that are compensation targets,calculates the voltage upper limit value and the current upper limitvalue in a descending order of the priorities.
 9. The power conversiondevice according to claim 1, wherein for a frequency component that is acompensation target, the harmonic compensation circuitry calculates avoltage upper limit value and a current upper limit value, extracts aminimum value among a first current value calculated from the voltageupper limit value and an impedance based on the frequency, the currentupper limit value, and a second current value based on an amplitude ofthe compensation current desired value, and divides the minimum value bythe second current value, to calculate the limit coefficient.
 10. Thepower conversion device according to claim 9, wherein the harmoniccompensation circuitry, with priorities set for respective frequencycomponents that are compensation targets, calculates the voltage upperlimit value and the current upper limit value in a descending order ofthe priorities.
 11. The power conversion device according to claim 10,wherein the control circuitry generates a first current command for afundamental positive-phase-sequence component, and adds the firstcurrent command and the compensation current command, to generate anoutput current command for the power converter, and the harmoniccompensation circuitry calculates the voltage upper limit value and thecurrent upper limit value for the frequency component of which thepriority is at a first rank, on the basis of the maximum voltage and themaximum current, and the first current command.
 12. The power conversiondevice according to claim 10, wherein the harmonic compensationcircuitry calculates the limit coefficients using the voltage upperlimit values and the current upper limit values only for the frequencycomponents of which the priorities are at a second rank or below, andcalculates the limit coefficient individually for the frequencycomponent of which the priority is at a first rank.
 13. The powerconversion device according to claim 12, wherein the harmoniccompensation circuitry calculates the voltage upper limit value and thecurrent upper limit value for the frequency component of which thepriority is at the second rank, on the basis of the maximum voltage andthe maximum current, and maximum current values respectively set for afundamental positive-phase-sequence component and the frequencycomponent of which the priority is at the first rank.
 14. The powerconversion device according to claim 10, wherein for each frequencycomponent that is the compensation target, the harmonic compensationcircuitry calculates voltage and current to be allocated for thefrequency component, and subtracts the calculated voltage and currentrespectively from the voltage upper limit value and the current upperlimit value, to calculate the voltage upper limit value and the currentupper limit value for the frequency component of which the priority isat a next rank.
 15. The power conversion device according to claim 10,wherein among the priorities set for the respective frequency componentsthat are the compensation targets, priorities for a plurality offrequency components at a second rank or below are set at the same rank,and for a group of the frequency components at the same rank of prioritythat are the compensation targets, the harmonic compensation circuitrycalculates the voltage upper limit value and the current upper limitvalue, on the basis of amplitudes of the compensation current desiredvalues for the group of frequency components, calculates the secondcurrent value which is an amplitude sum, and a third current value whichis a sum of the amplitudes imparted with weights based on the respectivefrequencies, extracts the minimum value among: the first current valuebased on the second current value, the third current value, the voltageupper limit value, and the impedance; the current upper limit value; andthe second current value, and divides the minimum value by the secondcurrent value, to calculate the limit coefficient.
 16. The powerconversion device according to claim 10, wherein the frequency componentof which the priority is at a first rank is a fundamentalnegative-phase-sequence component.
 17. The power conversion deviceaccording to claim 9, wherein the maximum voltage to be used forcalculation by the harmonic compensation circuitry is corrected byvoltage of the AC grid to which the power converter is connected. 18.The power conversion device according to claim 9, wherein the powerconverter is configured as a modular multilevel converter in which aplurality of phases each including a plurality of converter cells areconnected by single star connection, and the control circuitry controlsthe converter cells so as to output zero-phase-sequence voltage forinterphase balance, and the harmonic compensation circuitry uses thevoltage upper limit value corrected by the zero-phase-sequence voltage.19. The power conversion device according to claim 9, wherein the powerconverter is configured as a modular multilevel converter in which aplurality of phases each including a plurality of converter cells areconnected by delta connection, and the control circuitry controls theconverter cells so as to output circulation current for interphasebalance, and the harmonic compensation circuitry uses the current upperlimit value corrected by the circulation current.
 20. The powerconversion device according to claim 9, wherein the power converter isconfigured as a modular multilevel converter in which a plurality ofphases each including a plurality of converter cells are connected bydouble star connection, and the control circuitry controls the convertercells so as to output circulation current for interphase balance, andthe harmonic compensation circuitry uses the current upper limit valuecorrected by the circulation current.